The field of the invention relates to sorbents that may be utilized in desulfurization processes. In particular, the field of the invention relates to porous titanium dioxides that are impregnated with silver material.
The removal of sulfur compounds from hydrocarbon streams is an important part of petroleum refining operations. Sulfur is both an environmental pollutant when the hydrocarbon streams are utilized as a fuel for combustion as well as a poison for several catalytic materials when used in electrochemical systems such as a fuel cell.
The hydrodesulfurization (HDS) process is a widely used refining technique for the removal of sulfur compounds. However, HDS technology requires hydrogen gas, high pressures, and high temperatures, thus making the process energy intensive.
Adsorption of sulfur components from fuels is an alternative technique using sulfur-specific compositions such as molecular sieves or activated alumina as adsorbent materials for removing sulfur components from liquid hydrocarbon streams. The technique has several advantages over HDS, for example, where adsorption may be carried out at room temperature and atmospheric pressures. Furthermore, multiple cycles of adsorption and regeneration are possible using some of these adsorbents. (See, e.g., U.S. Pat. Nos. 7,148,389 and 6,531,052). Although HDS is effective in removing a large portion of the sulfur content in petroleum, some heavy heterocycles are not effectively removed, primarily due to the inactivity of the HDS catalyst towards these sulfur heterocyclic atoms. Adsorptive desulfurization is capable of removing these molecules selectively from liquid fuels. (See, e.g., U.S. Pat. Nos. 7,341,977 and 2,261,586). The adsorption may be carried out at ambient conditions of temperature and pressure without using any hydrogen. Thus, there may be substantial savings in operation cast for adsorptive systems over conventional HDS systems in some applications.
The affinity of silver for sulfur is evident from the process by which silver tarnishes, adsorbing hydrogen sulfide and other sulfur compounds from the ambient air to form a layer of sulfides on its surface. Silver metal has been utilized for the removal of sulfur in the prior art. (See, e.g., U.S. Pat. Nos. 4,188,285; 3,971,712; and 2,768,986). Sorbents having silver oxide supported on activated carbon, gamma alumina, or silica have been suggested for the removal of sulfur compounds from hydrocarbon streams. (See, e.g. U.S. Pat. No. 6,531,052; U.S. Pat. No. 4,738,771; U.S. Pat. No. 2,791,540). U.S. Pat. No. 6,271,173 and U.S. Published Application No. 20030183802 disclose a desulfurizing sorbent that may include reduced silver (among other metals) as an active desulfurizing agent supported on zinc oxide, silica, or alumina.
U.S. Pat. No. 4,430,205 describes the use of silver nitrate alone as the active sulfur removal component. However, silver nitrate may not be stable when used with complex fuels, and it is not possible to regenerate this composition in air due to nitrate decomposition. Furthermore, the preparation process disclosed in this patent takes more than eighteen hours. U.S. Pat. No. 4,582,819 describes the use of silver carbonate as an active sulfur removing component supported on alumina. The preparation process disclosed in this patent utilizes a mulling process which requires heating and forming a mixture of the carbonate precursor with the alumina powder, followed by palletizing, drying and calcining. This process is long and energy intensive and renders this composition very tedious to prepare on a large scale.
Clearly, better sorbent compositions, systems, and methods for removing sulfur compounds from hydrocarbon streams are desirable.